Virtual colonoscopy is a non-invasive method, in particular for examining the large intestine of a patient on the basis of image information relating to the large intestine, which image information was obtained, for example, by a magnetic resonance scanner or an X-ray computed tomography scanner. Prior to recording the image information relating to the large intestine, a contrast agent, e.g. air or carbon dioxide in the case of X-ray computed tomography imaging or water in the case of magnetic resonance imaging, is generally applied rectally into the large intestine of the patient so that the large intestine unfolds.
Furthermore, the large intestine is generally also freed of stool remains prior to the recording of image information, because only this allows meaningful diagnosis of the interior wall of the large intestine. However, patients often perceive precisely this removal of stool remains to be uncomfortable and so methods are preferred in which the removal of stool remains is avoided to a great extent or at least is less uncomfortable.
One method of avoiding too rigorous removal of stool remains from the large intestine prior to recording image information is the digital subtraction of marked stool from a volume data record having image information relating to the intestines. The stool remains are marked by a contrast agent, which is generally dispensed orally to the respective patient and can be easily detected in the utilized imaging method.
By way of example, barium was found to be a suitable contrast agent for X-ray computed tomography. The dispensed barium and the remaining stool mix in the large intestine until the recording of the image information. This procedure is also referred to as stool tagging, i.e. the labeling or marking of the stool. The stool-barium mixture has CT values of greater than 200 Hounsfield units in the volume data record of the imaging method, which values can be easily recognized. By way of example, the recognition can be brought about by means of a thresholding method. The image voxels representing the stool-barium mixture, identified with the aid of the thresholding method, ultimately obtain a CT value of approximately −1000 Hounsfield units, which corresponds to the CT value of the rectally administered air and is referred to as digital subtraction.
What are problematic in this method are, in particular, the transitions between stool-barium mixture and air and between stool-barium mixture and intestinal tissue. There are also transitions in the CT values here, and so the affected voxels cannot be assigned unambiguously to one of the following groups: intestinal tissue, stool-barium mixture or air. An additional difficulty is that the remaining stool and the barium often do not mix homogeneously.
Therefore, undesired structures that have the shape of intestinal polyps sometimes remain in the volume data record after the digital subtraction. Such structures remaining in the volume data record can accordingly lead to false diagnoses. Secondly, fine structures, such as intestinal folds, can be subtracted by mistake in the digital subtraction, and so a lesion, should it be located there, cannot be diagnosed.